1. Field of the Invention
This invention relates to semiconductor manufacturing and more particularly to a method and apparatus for controlling the bias voltage at a wafer in a plasma etch reactor and the de-coupling of the radio frequency (RF) in the case of use of multi-RF sources.
2. Description of the Related Art
Plasma etch reactors are currently available with a dual radio frequency (RF) system, where each RF system can operate at a different frequency. FIG. 1 illustrates diagram 100 depicting a prior art dual frequency RF system containing two electrodes. The reactor of diagram 100 contains an RF-driven electrode 104 and a passive electrode 102 with a wafer 116 disposed on top of the RF-driven electrode 104. The RF currents are supplied to the RF driven electrode via hard bolted metal connection 110. Upon activation of the RF power source, the currents enter the plasma region via the RF driven electrode and follow the same paths through the reactor to the passive electrode 102 as depicted by arrows 114. Additionally, the RF currents proceed through the plasma region to ground extensions 116 and 118 as illustrated by arrows 114a. Since the current paths of the 27 megahertz (MHz) and the 2 MHz are similar, the 2 MHz and the 27 MHz frequencies are not de-coupled.
The prior art plasma reactors can not independently control the plasma density and the ion energy of the plasma in the chamber of the reactor. Typically, the higher frequency of the dual RF system influences the plasma density, while the lower frequency influences the voltages in the reactor, in turn affecting the ion energy. However, since the dual frequencies are coupled, it is not possible to independently control plasma density and the ion energy. While the power supplied for one RF system of the dual frequencies may be modified to increase or decrease the plasma density, the modification also has an effect on the ion energy. That is to say, the vectors controlling the plasma density and the ion energy are not orthogonal in a system where both frequencies are coupled. Therefore, a change in one parameter, such as the plasma density, also affects the other parameter (ion energy) and vice versa.
Additionally, the ratio of the area of the passive electrode to the area of the RF-driven electrode determines the bias voltage at the wafer. For example, as the area of the RF-driven electrode decreases in relation to that of the passive electrode, the bias voltage at the RF-driven electrode will increase. Consequently, since the wafer sits on the RF-driven electrode, the bias voltage at the wafer of the ions increases. As a result, the energy impinging on the wafer during etch operations increases. While high ion energy resulting from a high bias voltage, may be beneficial for some etch processes, it is undesirable for others. For example, high ion energy is preferred for high aspect ratio etching, i.e., drilling deep small contact holes in a substrate. Other processes, such as etching a trench, however, are optimal at lower ion energies. Unfortunately, the bias voltage control at the wafer is limited due to the geometry of the plasma etch chamber in particular, due to the electrode area ratio.
While it is possible to transfer the wafer to a plasma reactor having a different geometry, i.e., smaller diameter passive electrode for processes requiring lower ion energies, there is the added expense of supplemental capital equipment and the risk of contamination of the wafer because of additional handling. Another alternative would be to replace the passive electrode with a smaller diameter electrode, however, this alternative again adds cost and only exacerbates the contamination and handling issues.
As a result, there is a need to solve the problems of the prior art to allow for greater independent control of plasma density and ion energy of the chamber to optimally accommodate various etch processes. Additionally, there is a need to control the bias voltage at the RF-driven electrode over a wide range in order to adapt the reactor so that it may be finely tuned for various etch processes.
Broadly speaking, the present invention fills these needs by providing a method and apparatus for controlling the bias voltage at the wafer over a broad range. Additionally, the invention provides a method and apparatus to de-couple dual RF currents (having different frequencies) thereby allowing for independent control of plasma density and ion energy of a plasma. It should be appreciated that the present invention can be implemented in numerous ways, including as an apparatus, a system, a device, or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, an apparatus for a plasma processing chamber is provided. In this embodiment, the processing chamber includes an RF-driven electrode configured to hold a substrate and a first and second radio frequency (RF) power supply connected to the RF-driven electrode. Also included is a passive electrode which is surrounded but electrically isolated from a ground extension. A filter array defining a set of filter settings is included. A switch is coupled to the top electrode and the switch is configured to interconnect the top electrode to one of the filter settings. The filter settings are configured to enable or disable RF current generated from one or both of the RF power supplies from passing through the top electrode.
In another embodiment of the invention an apparatus for an etch system having selectable modes of operation is provided. In this embodiment, an RF-driven electrode configured to accept a first and second RF current from a first and second RF generator is included. A passive electrode defined over the RF-driven electrode defining a plasma region therebetween and the passive electrode being electrically floating is included. Also included in this embodiment is a selector switch configured to enable passage of one both or none of the first and second RF currents through the passive electrode.
In yet another embodiment of the invention, an apparatus for de-coupling dual frequencies used in plasma etching is provided. In this embodiment, a plasma etch reactor is included. The plasma etch reactor includes a bottom and top electrode, a first and second RF power supply connected to the bottom electrode and a plasma region defined by the top and bottom electrode. The top electrode is electrically isolated from a top ground extension. A first RF return path to the first and second power supplies passes through the top electrode is included. A second RF current return path to the first and second power supplies passes through a ground extension is also included. A selector switch configured to block one both or none of the RF currents from the first RF current return path is included.
In still another embodiment of the invention, an apparatus for etching a substrate is provided. In this embodiment, a first and second power supply configured to deliver RF currents to a bottom electrode is included. A top electrode defining a first return path to the power supplies and a top and bottom ground extension defining a second and third return path to the power supplies are also included. A selector switch having selectable modes of operation where one, all or none of the RF currents are blocked from the first return path is included.
In still yet another embodiment, a method for etching a wafer in a plasma etch chamber is provided. The method includes providing an etch system having selectable modes of operation to process a substrate. Then, an etch recipe configured to set an etch environment in the etch system is selected. Next, one of the modes of operation to establish a selected bias voltage at the substrate is selected. Then, an etch occurring under the set etch environment and the selected bias voltage is performed.
The advantages of the present invention are numerous. Most notably, the apparatus and method allow control of the bias voltage at the substrate over a wide range. By selectively blocking RF currents from a return path, the effective area ratio of the top and bottom electrodes is modified, thereby influencing the bias voltage. Hence, the built-in bias voltage restrictions created by the design of the plasma etch chamber are circumvented. Additionally, the selective blocking of the RF currents allows for decoupling dual RF current supplied to the etch chamber. Therefore, a more independent control of the plasma density and the ion energy is achievable under these conditions.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.